Composite materials of metals and reinforcing carbon or Si-containing fibers are useful as light-weight structural materials having both the high elasticity, high strength and light weight of the carbon or Si-containing fibers and the shapability, heat stability, electrical conductivity and heat conductivity of the metals. In particular, the usefulness of SiC fibers as reinforcements in metal matrices such as aluminum and magnesium is recognized in the industry. However, the reactivity of SiC is sufficiently high to require the use of relatively low temperature or shorter processing time at high temperature during fabrication of the fiber reinforced metal composites in order to prevent fiber degradation. In addition, the reactivity of SiC limits the choice of metal matrix material and further, may well define the temperature to which the composite is limited in operation.
Carbon fibers have excellent strength and modulus of elasticity, but very bad wetting property with respect to a matrix such as molten aluminum. Furthermore, carbon fibers tend to react with molten aluminum at high temperatures and cause a drastic reduction in the strength of the resulting composite material. Thus, if the carbon fibers are directly used to reinforce composite materials, the resulting products cannot possess the desired mechanical strength.
It is known that many fiber-matrix combinations undergo extensive chemical reaction or interdiffusion between the fiber and matrix materials, each of which is likely chosen for the contribution of specific mechanical and/or physical properties to the resulting composite. Such reaction or interdiffusion can lead to serious degradation in strength, toughness and temperature stability. The fiber-matrix interface is therefore very important to preventing or minimizing chemical reactions and interdiffusion.
Surface modification of the fibers is an effective means to control the fiber-matrix interface. This can be accomplished by coating the fibers with a suitable composition to inhibit the fibers from reacting or bonding with the matrix.
A variety of coatings have been suggested for reinforcements intended for use in fiber-matrix composites. For example, U.S. Pat. No. 3,811,920 to Galasso et al. discusses applying a thin layer of TiC to a filamentary substrate having a SiC surface layer. The TiC layer is reported to impart oxidation resistance to the filament and the matrix metals. Boron nitride has also been used as a SiC coating, as in U.S. Pat. No. 4,642,271 to Rice.
U.S. Pat. No. 3,867,191 to Galasso et al. discloses coating a carbon filament with an amorphous carbon-boron alloy for use in a metal matrix.
U.S. Pat. No. 4,511,663 to Taylor discloses metal coated carbon fiber reinforcements which are used in a glass or glass-ceramic matrix.
Composite materials which have employed coatings such as the foregoing nonetheless remain limited for high temperature application because of concerns regarding the thermomechanical stability, thermochemical stability and high temperature fatigue resistance encountered at temperatures around 1000.degree. C.
It is an object of the present invention, therefore, to provide a coating for carbon or Si-containing reinforcements which permits the use of the reinforcement in metal composite materials for use at high temperatures.
Another object of the invention is to provide a coating for carbon or Si-containing reinforcement which prevents chemical reaction between the fiber and the matrix.